Apparatus and method for access to network via satellite

ABSTRACT

A system in which a personal computer sends messages into a TCP/IP network using a conventional dial-up link and downloads data from the TCP/IP network using a high-speed one-way satellite link. A preferred embodiment uses a conventional SLIP provider to connect to the TCP/IP network and uses a commercial software TCP/IP package that has a standard driver interface. A spoofing protocol compensates for the long propagation delays inherent to satellite communication.

BACKGROUND OF THE INVENTION

[0001] This application relates to a computer network and, morespecifically, to a method and apparatus for allowing both high-speed andregular-speed access to a computer network.

[0002] The Internet is an example of a TCP/IP network. The Internet hasover 10 million users. Conventionally, access to the Internet isachieved using a slow, inexpensive method, such as a terrestrial dial-upmodem using a protocol such as SLIP (Serial Line IP), PPP, or by using afast, more expensive method, such as a switched 56 Kbps, frame relay,ISDN (Integrated Services Digital Network), or T1.

[0003] Users generally want to receive (download) large amounts of datafrom networks such as the Internet. Thus, it is desirable to have aone-way link that is used only for downloading information from thenetwork. A typical user will receive much more data from the networkthan he sends. Thus, it is desirable that the one-way link be able tocarry large amounts of data very quickly. What is needed is a highbandwidth one-way link that is used only for downloading information,while using a slower one-way link is used to send data into the network.

[0004] Currently, not all users have access to high speed links tonetworks. Because it will take a long time to connect all users tonetworks such as the Internet via physical high-speed lines, such asfiber optics lines, it is desirable to implement some type of high-speedline that uses the existing infrastructure.

[0005] Certain types of fast network links have long propagation delays.For example, a link may be transmitting information at 10 Mbps, but itmay take hundreds of milliseconds for a given piece of information totravel between a source and a destination on the network. In addition,for even fast low-density links, a slow speed return-link may increasethe round trip propagation time, and thus limit throughput. The TCP/IPprotocol, as commonly implemented, is not designed to operate over fastlinks with long propagation delays. Thus, it is desirable to take thepropagation delay into account when sending information over such alink.

SUMMARY OF THE INVENTION

[0006] The present invention overcomes the problems and disadvantages ofthe prior art by allowing a user to download data using a fast one-waysatellite link, while using a conventional low-speed Internet connectionfor data being sent into the network. The invention uses a “spoofing”technique to solve the problem of the long propagation delays inherentin satellite communication.

[0007] In accordance with the purpose of the invention, as embodied andbroadly described herein, the invention is a network system that forms apart of a network, comprising: a source computer, having a link to thenetwork; a destination computer, having a link to the network; asatellite interface between the source computer and the destinationcomputer, wherein information passes from the source computer to thedestination computer; means in the destination computer for requestinginformation from the source computer over the network; means forreceiving an information packet sent from the source computer inresponse to the request and for sending the information packet to thedestination computer over the satellite interface; and means for sendingan ACK message to the source computer in response to receipt of theinformation packet, wherein the ACK message appears to the sourcecomputer to have come from the destination computer.

[0008] In further accordance with the purpose of the invention, asembodied and broadly described herein, the invention is a gateway in anetwork system that forms a part of a TCP/IP network, wherein thenetwork includes a source computer having a link to the TCP/IP networkand a link to a high speed satellite interface, and a destinationcomputer having a link to the TCP/IP network and a link to the highspeed satellite interface, the gateway comprising: means for receivingan information packet sent from the source computer and for sending theinformation packet to the destination computer over the satelliteinterface; and means for sending an ACK message to the source computerin response to receipt of the information packet, wherein the ACKmessage appears to the source computer to have come from the destinationcomputer.

[0009] Objects and advantages of the invention will be set forth in partin the description which follows and in part will be obvious from thedescription or may be learned by practice of the invention. The objectsand advantages of the invention will be realized and attained by meansof the elements and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The accompanying drawings, which are incorporated in andconstitute a part of this specification, illustrate several embodimentsof the invention and, together with the description, serve to explainthe principles of the invention.

[0011]FIG. 1 is a hardware block diagram of a preferred embodiment ofthe invention;

[0012]FIG. 2 is a diagram of a portion of a hybrid terminal of FIG. 1;

[0013]FIG. 3 is a diagram showing an IP packet format;

[0014]FIG. 4 is a diagram showing a plurality of packet formats,including an Ethernet packet format;

[0015]FIG. 5 is a diagram showing a tunnelling packet format;

[0016]FIG. 6 is a diagram of steps performed by the hybrid terminal ofFIG. 1;

[0017]FIG. 7 is a diagram showing an example of partial data in atunnelling packet;

[0018]FIG. 8 is a flowchart of steps performed by the hybrid terminal ofFIG. 1;

[0019]FIG. 9 is a diagram of steps performed by a hybrid gateway of FIG.1;

[0020]FIG. 10 is a diagram showing a format of packets sent to asatellite gateway of FIG. 1;

[0021]FIG. 11 is a diagram showing a TCP packet format;

[0022]FIG. 12 is a ladder diagram showing packets sent from anapplication server to the hybrid gateway and from the hybrid gateway tothe hybrid terminal over a satellite link; and

[0023] FIGS. 13(a) through 13(e) are flowcharts of steps performed bythe hybrid gateway of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Reference will now be made in detail to the preferred embodimentsof the invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

[0025] a. General Overview

[0026] A preferred embodiment of the present invention uses satellitetechnology to implement a high-speed one way link between a user'scomputer and a TCP/IP network, such as the Internet or a private TCP/IPnetwork. This high-speed link is used to download data from the network.The user's computer also has a conventional TCP/IP link for sending datato the network. The invention can use various forms of high-speed,one-way links, such as satellites, and cable television lines. Theinvention can use various forms of low-speed networks, such as TCP/IPnetworks, dialup telephones, ISDN D-channel, CPDP, and low-speedsatellite paths.

[0027] The described embodiment of the present invention uses satellitesto provide a high-speed one-way link. Satellites can cover largegeographical areas and are insensitive to the distance between atransmitter and a receiver. In addition, satellites are very efficientat point-to-point and broadcast applications, and are resilient andresistant to man-made disasters. Two-way satellites are expensive touse, however, because of the costs involved in purchasing and installingsatellite earth station hardware. In the past, these costs have placedsatellite communications outside the reach of the consumer.

[0028] The present invention allows a personal computer to receivedownloaded information from the network via a satellite at a verypractical cost. In the present invention, the cost of satellitecommunications is reduced because a one-way satellite link is used.Receive-only earth station equipment is cheaper to manufacture becauseit requires less electronics than send/receive antennae.

[0029] As is well-known in the art, communication over the Internet andsimilar TCP/IP networks is achieved through a group (suite) of protocolscalled Transmission Control Protocol/Internet Protocol (TCP/IP). TheTCP/IP protocol is described in the book “Internetworking With TCP/IP,Vol I” by Douglas Comer, published by Prentice-Hall, Inc., of EnglewoodCliffs, N.J., 1991, which is incorporated by reference.

[0030] b. Hybrid TCP/IP Access

[0031]FIG. 1 is a hardware block diagram of a preferred embodiment ofthe invention. FIG. 1 includes five subsystems: a hybrid terminal 110, aSLIP provider (Internet connection) 130, an application server 140, ahybrid gateway 150, and a satellite gateway 160. Hybrid terminal 110 isconnected to a modem 190, e.g., a 9600 baud modem, which connects toSLIP provider 130 through a telephone line 192. A satellite transmitter170, a satellite 175, and a satellite receiver 180 provide a fast,one-way link for transferring data from satellite gateway 160 to hybridterminal 110. Each of SLIP provider 130, application server 140, andhybrid gateway 150 are connected to the Internet 128. As is well-knownin the art, the Internet 128 is a “network of networks” and can bevisually depicted only in general terms, as seen in FIG. 1.

[0032] Each of hybrid terminal 110, SLIP provider 130, applicationserver 140, hybrid gateway 150 and satellite gateway 160 includes aprocessor (not shown) that executes instructions stored in a memory (notshown). Other parts of the invention also include processors that arenot discussed herein, such as I/O processors, etc. Preferably, hybridterminal 110, hybrid gateway 150, and satellite gateway 160 areimplemented as personal computers including an 80386/80486 basedpersonal computer operating at least 33 MHz, but these elements can beimplemented using any data processing system capable of performing thefunctions described herein. In the described embodiment, SLIP provider130 is a conventional SLIP provider and application server 140 is anyapplication server that can connect to the Internet via TCP/IP.

[0033] As shown in FIG. 1, hybrid terminal 110 preferably includesapplication software 112, driver software 114, a serial port 122 forconnecting hybrid terminal 110 to modem 190, and satellite interfacehardware 120 for connecting hybrid terminal 110 to satellite receiver180.

[0034]FIG. 2 shows a relationship between software in application 112,software in driver 114, serial port 122, and satellite interface 120.Application software 112 includes TCP/IP software, such as SuperTCP,manufactured by Frontier, Inc., Chameleon, manufactured by Netmanager,and IRNSS, manufactured by Spry, Inc. The described embodimentpreferably operates with the SuperTCP TCP/IP package and, thus, uses astandard interface 212 between the TCP/IP software 210 and driver 114.Examples of standard interfaces 212 between TCP/IP software 210 anddriver 114 include the Crynson-Clark Packet Driver Specification and the3Com/Microsoft Network Driver Interface Specification (NDIS). Otherembodiments use other standard or non-standard interfaces between TCP/IPsoftware 210 and driver 114.

[0035] As shown in FIG. 2, application software 112 also includeswell-known Internet utilities, such as FTP, and well-known userinterfaces, such as Mosaic and Gopher (shown). Application software 112can also include other utilities, e.g., News and Archie (not shown).

[0036] The following paragraphs describe how a request from hybridterminal 110 is carried through the Internet 128 to application server140 and how a response of application server 140 is carried back to theuser at hybrid terminal 110 via the satellite link. The operation ofeach subsystem will be described below in detail in separate sections.

[0037] In the present invention, hybrid terminal 110 is given two IPaddresses. One IP packet address corresponds to SLIP provider 130 and isassigned by a SLIP service provider. The other IP address corresponds tosatellite interface 120 and is assigned by a hybrid service provider. IPaddresses are assigned by the SLIP and satellite network managers andloaded into hybrid terminal 110 as part of an installation configurationof the hybrid terminal's hardware and software. These two IP addressescorrespond to completely different physical networks. SLIP provider 130does not “know” anything about the satellite IP address or even whetherthe user is using the satellite service. If a host somewhere in theInternet is trying to deliver a packet to the satellite IP address byusing the Internet routing scheme of routers, gateways, and ARPs(Address Resolution protocol), the only way that the packet can reachthe satellite IP interface is to traverse the satellite by being routedthrough satellite gateway 160.

[0038] The following example assumes that a user at hybrid terminal 110desires to send a request to a remote machine, such as applicationserver 140, that is running FTP (File Transfer protocol) serversoftware. The FTP software running on application server 140 receivesfile transfer requests and responds to them in an appropriate fashion.

[0039]FIG. 1 shows the contents of a source field (SA) and of adestination field (DA) of packets sent between the elements of FIG. 1. Arequest for a file and a response of a file sent from application server140 to hybrid terminal 110 takes the following path.

[0040] 1) Within hybrid terminal 110, FTP client software 230 generatesa request and passes it to TCP/IP software 210. TCP/IP software 210places the request in a TCP packet (see FIG. 11). Next, the TCP packetis placed in an IP packet, having a format shown in FIG. 3. TCP/IPsoftware 210 places the IP packet in an Ethernet packet, as shown inFIG. 4, and passes the Ethernet packet to driver 114. This packet has asource IP address corresponding to satellite interface 120 and adestination IP address of application server 140.

[0041] 2) In driver 114, the Ethernet header and checksum are strippedoff the packet and the IP packet is encapsulated, or “tunneled,” insideof another IP packet and sent over serial port 122 to SLIP provider 130.FIG. 5 shows a format of a tunnelled packet. FIG. 7 shows an example ofa tunnelled packet. The encapsulation adds a new IP header 530 in frontof the original packet 540 with a source address corresponding to SLIPprovider 130 and a destination address corresponding to hybrid gateway150.

[0042] 3) SLIP provider 130 receives the IP packet, analyzes thetunneling header and, thinking it is destined for hybrid gateway 150,uses standard Internet routing to send the packet to hybrid gateway 150.

[0043] 4) When hybrid gateway 150 receives the packet, it strips off thetunneling header, revealing the true header with application server 140as the destination. The packet is then sent back out into the Internet128.

[0044] 5) Internet routing takes the packet to application server 140,which replies with the requested file and addresses the reply to therequest's source IP address, i.e., the IP address of the hybridterminal's satellite interface 120.

[0045] 6) In order to find the hybrid terminal's satellite interface120, the Internet routing protocol will send the packet to the subnetcontaining a router/gateway connected to hybrid gateway 150. When arouter on the same physical network as satellite gateway 160 and hybridgateway 150 sends out an ARP for the IP address of satellite interface120 (to find a physical address of satellite interface 120), hybridgateway 150 responds and says “send it to me.” Thus, application server140 and the rest of the Internet 128 thinks that packets sent to hybridgateway 150 will reach the hybrid terminal's satellite interface.

[0046] 7) Once hybrid gateway 150 receives a reply packet fromapplication server 140, it sends it to satellite gateway 160. In thedescribed embodiment, hybrid gateway 150 encapsulates the packet in aspecial packet format that is used over the satellite link and uses thesatellite interface IP address to uniquely identify the satellitepacket's destination. Then hybrid gateway 150 sends the packet over theEthernet to satellite gateway 160.

[0047] 8) Satellite gateway 160 broadcasts over the satellite link anypackets it receives from hybrid gateway 150.

[0048] 9) Driver 114 in hybrid terminal 110 that services satelliteinterface 120 scans all packets broadcast over satellite transmitter 170looking for its satellite interface IP address in the header. Once itidentifies one, it captures it, strips off the satellite headerrevealing the reply IP packet, and sends it to driver 114.

[0049] Thus, IP packets sent into Internet 128 are carried by the SLIPconnection, while IP packets from the Internet 128 are carried by thesatellite link. The following paragraphs describe the operation of eachsubsystem in more detail.

[0050] 1. The Hybrid Terminal

[0051] Hybrid terminal 110 is the terminal with which the userinteracts. Thus, hybrid terminal 110 includes a user interface device(not shown) such as a mouse, keyboard, etc. As shown in FIG. 1, hybridterminal 110 includes one or more application programs 112 (includingTCP/IP software 210), and driver software 114, which communicates withSLIP provider 130 through a serial port 122 and modem 190, using adriver portion 118, and which communicates with satellite receiver 180through a satellite interface 120, using a driver portion 116.

[0052] To TCP/IP software 210, driver 114 appears to be an Ethernetcard, although driver 114 is actually connected to satellite receiver180 (via satellite interface 120) and to SLIP provider 130 (via serialline 122 and modem 190). Thus, TCP/IP software 210 believes that it iscommunicating with a single physical network, when it is, in reality,communicating with two physical networks (the SLIP dialup network and asatellite network). Ethernet is a packet switching protocol standardizedby Xerox Corporation, Intel Corporation, and Digital EquipmentCorporation, which is described in “The Ethernet: A Local Area NetworkData Link Layer and Physical Layer Specification,” September 1980, whichis available from any of these three companies, and which isincorporated by reference.

[0053]FIG. 6 is a diagram of steps performed by driver 114 of hybridterminal 110 of FIG. 1. As shown in FIG. 6, driver 114 receives packetsof data from TCP/IP software 210 and passes them to SLIP provider 130via serial port 122 and modem 190. A packet sent by application server140 is received through satellite receiver 180, passed through thesatellite interface 120, passed to the satellite driver 220, and passedto driver 114, which passes the received packet to TCP/IP software 210.

[0054] The following paragraphs discuss two basic functions performed bydriver 114 (tunnelling and ARP handling) and discuss variousimplementation details for the described embodiment.

[0055] A. “Tunnelling”

[0056] As discussed above, hybrid terminal 110 has two IP addressesassociated with it: one for SLIP provider 130 and one for the satelliteinterface. Packets containing requests are sent from hybrid terminal 110to application server 140 via the Internet 128, while packets containinga reply are sent back via the satellite link. Tunnelling is the methodby which application server 140 is “fooled” into sending a reply to adifferent IP address (satellite interface 120) than that of the sender's(serial port 122).

[0057] A packet received by driver 114 from the TCP/IP software 210 hasa source address of satellite gateway 160 and a destination address ofapplication server 140. As shown in step 610 of FIG. 6, driver 114removes the Ethernet header and checksum and encapsulates the IP headerinto an IP tunnelling header having a source address of SLIP provider130 and a destination address of hybrid gateway 150 (see FIG. 7). Asdescribed above, at hybrid gateway 150, the tunnelling header is removedand the packet is sent back into the Internet 128 to be sent toapplication server 140.

[0058] When forming a tunnelling header, driver 114 copies all thevalues from the old header into the new one with the followingexceptions. The source and destination addresses of the tunnellingheader change, as described above. In addition, a total packet lengthfield 510 is changed to contain the contents of length field 310 plusthe length of the tunnelling header. Lastly, the driver 114 recalculateschecksum 520 of the tunnelling header because some of the fields havechanged.

[0059] B. ARP Handling

[0060] ARP (Address Resolution Protocol) is used by TCP/IP todynamically bind a physical address, such as an Ethernet address, to anIP address. When TCP/IP finds an IP address for which it does not know aphysical address, TCP/IP broadcasts an ARP packet to all nodes,expecting a response that tells TCP/IP what physical address correspondsto the IP address.

[0061] During initialization, driver 114 declares to TCP/IP software 210that driver 114 is an Ethernet card to ensure that the packets thatTCP/IP package sends are Ethernet packets and that the TCP/IP packagewill be prepared to receive packets at a high-rate of speed. As shown instep 620 of FIG. 6, when driver 114 detects that TCP/IP has sent an ARPpacket, driver 114 creates a physical address and sends a reply packetto TCP/IP software 210. The contents of the physical address areirrelevant, because driver 114 strips off the Ethernet header on packetsfrom TCP/IP before the packets are sent to SLIP provider 130.

[0062] C. Other Functions

[0063] As shown in step 630 of FIG. 6, packets received by driver 114from satellite receiver 180 (via satellite driver 114) are merely passedto TCP/IP software 210. The following paragraphs discuss implementationdetails for the described embodiment.

[0064] In a preferred embodiment, TCP/IP software 210 (e.g., Frontier'sSuperTCP) sends an ACK (acknowledge) for every packet it receives, eventhough this action is not required by the TCP/IP protocol. In thissituation, many packets compete for the slow link to SLIP provider 130.In TCP/IP, the ACK scheme is cumulative. This means that when atransmitter receives an ACK stating that the receiver has received apacket with sequence number N, then the receiver has received allpackets with sequence numbers up to N as well, and there is no reasonwhy every packet needs to be ACK'ed.

[0065]FIG. 8 is a flowchart of steps performed in a preferred embodimentby driver 114 of hybrid terminal 110. FIG. 11 is a diagram showing a TCPpacket format. FIG. 11 includes a sequence number field 1102, anacknowledgment (ACK) number field 1104, and a checksum field 1106. Instep 810 of FIG. 8, driver 114 receives an ACK packet with sequencenumber N from TCP/IP software 210. The packet is queued along with otherpackets waiting to be sent to SLIP provider 130. In step 820 driver 114checks to determine whether there is a “run” of sequential packetswaiting to be sent. If so, in step 830, driver 114 deletes ACK packetsfor the same TCP connection that have sequence numbers in the run fromthe queue and sends an ACK only for the highest sequence number in therun. This action alleviates the bottleneck caused by the relatively slowmodem speeds.

[0066] Serial port 122 provides a physical connection to modem 190 and,through it, to the terrestrial network via a SLIP protocol as describedbelow in connection with SLIP provider 130. Serial data is sent andreceived through an RS-232 port connector by UART (UniversalAsynchronous Receiver Transmitter), such as a U8250, which has a onebyte buffer and is manufactured by National Semiconductor, or a U16550,which has a 16 byte buffer and is also manufactured by NationalSemiconductor.

[0067] The invention preferably operates under the DOS operating systemand Windows, but also can operate under other operating systems.

[0068] Satellite driver software 220 receives packets from satellite180, and passes them to driver 114 using a DOS call. Thus, the twophysical links are combined within driver 114 and the existence of twophysical links is transparent to TCP/IP software 210. Satellite driver220 scans all packets transmitted over the satellite channel for apacket with a header corresponding to the IP address of the satelliteinterface 122, performs some error detection and correction on thepacket, buffers the received packet, and passes the packet to driver 114using a DOS call, e.g., IOCTL-output-cmd( ). Driver 114 copies data fromsatellite driver 220 as quickly as possible and passes it to TCP/IPsoftware 210.

[0069] As discussed above, TCP/IP software 210 is fooled into thinkingthat it is connected to an Ethernet network that can send and receive at10 Mbps. This concept is helpful on the receive side because data fromthe satellite is being received at a high rate. On the transmit side,however, modem 190 is not capable of sending at such a high rate. Inaddition, TCP/IP software 210 sends Ethernet packets to driver 114,i.e., an IP packet is encapsulated into an Ethernet packet. Because SLIPprovider 130 expects IP packets, driver 114 must strip the Ethernetheader before the packet is sent to SLIP provider 130.

[0070] As described above in connection with FIG. 8, Driver 114 alsoincludes a transmit and receive queue. As data is received from TCP/IPsoftware 210 and received from the satellite driver 220, it is bufferedwithin the queue. When the queue is full, e.g., when TCP/IP is sendingpackets faster than modem 190 can send them, driver 114 drops thepackets and returns an error so that TCP/IP software 210 will decreaseits rate of transmission.

[0071] In a first preferred embodiment, a SLIP connection is initiatedwith an automatic logon procedure. In another preferred embodiment,driver 114 executes instructions to allow a user to perform a SLIP logonmanually.

[0072] Because TCP/IP software 210 preferably is configured to talk toEthernet and it is desirable to receive the largest packet sizepossible, driver 114 configures TCP/IP so that the MTU (MaximumTransmission Unit) of the network is as large as possible, e.g., 1500bytes. Some SLIP providers 130 have a smaller MTU, e.g., 512 bytes. Tohandle the disparity in size, driver 114 segments large packets receivedfrom TCP/IP software 210 into segments the size of the SLIP MTU. Once apacket is segmented, it is reassembled in hybrid gateway 150. Only thetunnelling header is copied as the header of the segments.

[0073] 2. The SLIP Provider

[0074] SLIP provider 130 performs the function of connecting hybridterminal 110 to the Internet 128. As described above, other protocols,such as PPP, could also be used to perform the connecting function. SLIPserver 130 receives SLIP encoded IP packets from modem 190, uncodesthem, and forwards them to hybrid gateway 150 via the Internet 128.

[0075] In its most basic form, SLIP provider 130 delimits IP packets byinserting a control character hex 0xC0 between them. To insure that adata byte is not mistaken for the control character, all outgoing datais scanned for instances of the control character, which is replaced bya two character string. The SLIP protocol is described in detail in J.Romkey, “A Nonstandard for Transmission of IP Datagrams over SerialLines: SLIP,” RFC 1055, June 1988, pp. 1-6, which is incorporated byreference.

[0076] 3. The Application Server

[0077] Application server 140 is a computer system running anycombination of known application programs available on the Internetusing the TCP/IP protocol suite. For example, application server 140 maybe transferring files to requesting users via FTP. Although hybridterminal 110 actually has two IP addresses (a serial port address and anaddress for the satellite interface), the software executing onapplication server 140 thinks that it is receiving requests over thesatellite network and sending responses over the satellite network.Hybrid terminal 110 is completely transparent to application server 140.

[0078] 4. The Hybrid Gateway

[0079] Although only one hybrid terminal 110 is shown in FIG. 1, theinvention can include a plurality of hybrid terminals 110. Preferably,all packets sent from all hybrid terminals 110 pass through hybridgateway 150 to get untunnelled. Thus, hybrid gateway 150 is a potentialsystem bottleneck. Because of this potential bottleneck, the functionsof hybrid gateway 150 are as simple as possible and are performed asquickly as possible. Hybrid gateway 150 also has good Internetconnectivity to minimize the accumulated delay caused by packets waitingto be processed by hybrid gateway 150.

[0080] A. Untunnelling

[0081]FIG. 9 is a diagram of steps performed by hybrid gateway 150 ofFIG. 1. In step 910, hybrid gateway 150 receives a tunnelled packethaving a format shown in FIG. 5. Hybrid gateway 150 “untunnels” thepacket by stripping off the tunnelling header and passes the packet backto the Internet.

[0082] As described above, packets are sometimes broken into segmentswhen they are sent in order to accommodate a small MTU of SLIP provider130. Packets may also be segmented as they pass through other elementsof the net having small MTUs. For fragmented packets, only the tunnelledheader is copied into the header of each segment. Hybrid gateway 150stores fragmented packets in a memory (not shown) and reassembles themin order before untunnelling the original packet and passing it to theInternet. Preferably, a “time to live” value is assigned to each packetwhen it is sent by driver 114 and if all segments do not arrive before atime to live timer expires, the packet is discarded.

[0083] B. ARP Responding

[0084] Preferably, satellite gateway 160 is on a same physical networkas hybrid gateway 150. As shown in step 920 of FIG. 9, when a router onthe same physical network as satellite gateway 160 and hybrid gateway150 sends out an ARP for the IP address of satellite gateway 160 (tofind a physical address of satellite gateway 160), hybrid gateway 150responds and says “send it to me.” Hybrid gateway 150 needs to interceptpackets intended for satellite gateway 160 because it needs toencapsulate packets for satellite gateway 160 as follows.

[0085] C. Satellite Packetizing

[0086] The following paragraphs describe how packets travel fromapplication server 140 through hybrid gateway 150 and to satellitegateway 160. The following explanation is given by way of example and isnot intended to limit the scope of the present invention. As shown instep 930 of FIG. 9, hybrid gateway 150 encapsulates replies fromapplication server 140 into a satellite packet format. FIG. 10 is adiagram showing a format of a satellite packet sent to satellite gateway160 of FIG. 1. A satellite packet includes the data 1010 of an originalIP packet and two headers added by hybrid gateway 150.

[0087] Satellite gateway 160 expects IP packets to be encapsulated firstin a special satellite packet and then within an LLC-1 IEEE 802.2 linklevel control, type 1 packet. Satellite header 1020 identifies thedownlink and contains a sequence number and the packet length. An LLC-1header 1030 preferably is used to send the packet to satellite gateway160, in an Ethernet LAN. Hybrid gateway 150 prepares packets forsatellite gateway 160 by appending headers 1020 and 1030 to the front ofan IP packed 1010.

[0088] The receiver in hybrid terminal 110 does not receive the LLC-1header 1030. Hybrid terminal 110 identifies packets intended for it bychecking a least significant byte in the satellites IP address. Thus, asix byte satellite destination address is determined by reversing anorder of bytes of the satellite IP address for hybrid terminal 110 andthen padding the rest of the address with zeroes.

[0089] 5. The Satellite Gateway

[0090] Satellite gateway 160 can include any combination of hardware andsoftware that connects satellite transmitter 170 to hybrid gateway 150.Satellite transmitter 170 and satellite receiver 180 can be anycombination of hardware and software that allows data to be transmittedby satellite transmitter 170 and received by satellite receiver 180, andto be input to hybrid terminal 110. For example, satellite gateway 160preferably is a personal computer with a high-speed Ethernet connectionto hybrid terminal 110. When satellite gateway 160 receives a packetfrom hybrid gateway 150, it sends it over the satellite link.

[0091] Satellite communication may be effected by, for example, thePersonal Earth station manufactured by Hughes Network Systems Corp. In apreferred embodiment, a one-way version of the Personal Earth Station isused. Another embodiment uses a satellite communication systemmanufactured by Comstream. Yet another embodiment uses a system thatallows hybrid terminal 110 to be connected directly to satellitereceiver 180 via Hughes Network Systems' DirectPC product. The DirectPCsatellite interface card is described in “DirectPC, Phase A Data Sheet,”dated Jun. 7, 1993, which is attached as Appendix A, which isincorporated in and constitutes a part of this specification, and whichis incorporated by reference.

[0092] At the downlink, satellite receiver 180 includes a 0.6 meterreceive-only antenna receiving HDLC encapsulated LAN packets. Satelliteinterface 120 includes rate ⅔ Viterbi/Reed-Soloman concatenated forwarderror correction.

[0093] Although only one hybrid terminal 110 and one application server140 are shown in FIG. 1, the invention can include a plurality of hybridterminals 110 and/or a plurality of application servers 140. Preferably,all packets sent from all application servers 140 to a hybrid interface110 pass through satellite gateway 160. Thus, satellite gateway 160 is apotential system bottleneck. Because of this potential bottleneck, thefunctions of satellite gateway 160 are as simple as possible and areperformed as quickly as possible.

[0094] c. Protocol Spoofing

[0095] TCP/IP protocol specifies that only a predetermined number ofpackets can be outstanding during transmission, i.e., that only alimited number of packets can be sent before an ACK (acknowledgment) isreceived. The high bandwidth and long delays incurred in sending packetsto an orbiting satellite and back means that at any given time, a lot ofpackets are “in the pipe” between transmitter and receiver.

[0096] When using conventional TCP/IP protocol, application server 140sends a predetermined number of packets in accordance with apredetermined window size, and then waits to receive ACKs over the modemlink before sending additional packets. The purpose of windowing is tolimit a number of packets that must be re-sent if no ACK is received andto provide flow control, e.g., to prevent sending packets faster thanthey can be received. The packets that have not been ACK'ed are storedin a memory so that they can be re-sent if no ACK is received.

[0097] In a preferred embodiment of the present invention, hybridgateway 150 “spoofs” application server 140 to improve the throughputover the satellite link. Specifically, hybrid gateway 150 sends an ACKto application server 140, even though a corresponding packet may nothave been received by hybrid terminal 110 via the satellite at the time.

[0098]FIG. 12 is a ladder diagram showing packets sent from applicationserver 140 to hybrid gateway 150 and from hybrid gateway to hybridterminal 110 through the satellite link. FIG. 12 is not drawn to scale.In FIG. 12, application server 140 sends a message #1 to hybrid gateway150. The propagation time for this transmission is relatively short.Hybrid gateway 150 immediately creates an ACK packet and sends it toapplication server 140. Hybrid gateway also sends packet #1 to hybridterminal 110 through the satellite link. This transmission has a longpropagation delay. When hybrid terminal 110 receives the packet, itsends an ACK #1 back to hybrid gateway 150 (e.g., using the tunnellingmechanism described above). In a system that does not use tunnelling,hybrid gateway 150 needs to intercept the ACK packets from hybridterminal 110.

[0099] FIGS. 13(a) through 13(e) are flowcharts of steps performed byhybrid gateway 150 of FIG. 1 during protocol spoofing. In step 1302 ofFIG. 13(a), hybrid gateway 150 receives a packet from application server140 indicating that a new connection is being formed between applicationserver 140 and hybrid terminal 110. In step 1304, hybrid gateway 150sets up a queue or similar data structure in memory to save un-ACK'edpackets for the new connection. FIG. 13(b) show corresponding stepsperformed by hybrid gateway 150 when the connection is closed. Hybridgateway 150 receives a packet indicating the closure in step 1306 anddeletes the queue and saved values for the connection in step 1308.

[0100] In step 1310 of FIG. 13(c), hybrid gateway 150 fails to receivean ACK for a packet number X from hybrid terminal 110 before an end of apredetermined timeout period. Hybrid gateway 150 maintains a timer foreach un-ACK'ed packet. At the end of the predetermined period, hybridgateway 150 retransmits a packet corresponding to the expired timer. Instep 1312, hybrid gateway 150 re-sends packet number X, which itpreviously saved in the memory queue for this connection (see FIG. 13(d)below).

[0101] In step 1314 of FIG. 13(d), hybrid gateway 150 receives a packetfrom application server 140. In step 1316, hybrid gateway 150 sends thereceived packet to satellite gateway 160, where it is transmitted overthe satellite link, and saves the packet in case it needs to beretransmitted (see FIG. 13(c)). Hybrid gateway 150 then creates an ACKpacket to send to application server 140 in step 1318. The created ACKpacket incorporates a format shown in FIG. 11. Hybrid gateway 150creates an ACK number for field 1104. The ACK number is determined asfollows:

[0102] Hybrid gateway 150 saves the following information for eachconnection:

[0103] 1) Send sequence number—a highest in-sequence sequence number ofpackets sent by application server 140 over the connection.

[0104] 2) ACK sequence number—the ACK sequence number from the mostrecent packet sent by hybrid terminal 110 over this connection.

[0105] 3) ACK window size—the window size from the most recent packetfrom hybrid terminal 110 over this connection.

[0106] 4) ACK number—the ACK sequence number that is relayed toapplication server 140. The ACK number is set to:

minimum(send sequence number, ACK sequence number+spoofed windowsize−ACK window size).

[0107] 5) spoofed window size—predetermined maximum number window sizeto be allowed on this connection.

[0108] When hybrid gateway 150 inserts the ACK number in the packet, italso calculates the packet's checksum 1106.

[0109] In step 1320 of FIG. 13(e), hybrid gateway 150 receives an ACKpacket over the modem link from hybrid terminal 110. In step 1322,hybrid gateway 150 removes from the queue the packet for which the ACKwas received. Because an ACK was received, the packet does not need tobe re-sent. In the TCP/IP protocol, a packet containing an ACK may ormay not contain data. Hybrid gateway 150 edits the received packet toreplace the packet's ACK number 1104 with a “spoofed” ACK number in step1326. The spoofed ACK number is determined in the same way as the ACKnumber in step 1318 of FIG. 13(d). When hybrid gateway 150 substitutesthe spoofed ACK number 1104 in the packet, it also recalculates thepacket's checksum 1106 in step 1326.

[0110] In step 1328, hybrid gateway 150 forwards the received ACK packetto application server 140. Application server 140 may simply disregardthe packet if it contains an ACK and no data. In another embodiment,hybrid gateway 150 simply discards a packet received from hybridterminal 110 that contains an ACK, but no data.

[0111] If the connection goes down, either explicitly or after apredetermined period of time, hybrid gateway 150 deletes the savedpackets for the connection.

[0112] d. Summary

[0113] In summary, the present invention allows a personal computer tosend messages into the Internet using a conventional dial-up link and todownload data from the Internet using a high-speed one-way satellitelink. In a preferred embodiment, the invention uses a conventional SLIPprovider to connect to the Internet and uses a commercial softwareTCP/IP package that has a standard driver interface. A spoofing protocolcompensates for the long propagation delays inherent to satellitecommunication.

[0114] Other embodiments will be apparent to those skilled in the artfrom consideration of the specification and practice of the inventiondisclosed herein. It is intended that the specification and examples beconsidered as exemplary only, with a true scope of the invention beingindicated by the following claims.

What is claimed is:
 1. A network system that forms a part of a network,comprising: a source computer, having a link to the network; adestination computer, having a link to the network; a satelliteinterface between the source computer and the destination computer,wherein information passes from the source computer to the destinationcomputer; means in the destination computer for requesting informationfrom the source computer over the network; means for receiving aninformation packet sent from the source computer in response to therequest and for sending the information packet to the destinationcomputer over the satellite interface; and means for sending an ACKmessage to the source computer in response to receipt of the informationpacket, wherein the ACK message appears to the source computer to havecome from the destination computer.
 2. The network of claim 1, furthercomprising means for receiving a packet containing an ACK message fromthe destination computer, for indicating receipt of the informationpacket by the destination computer, and for discarding the ACK messagereceived from the destination computer when no other data is present inthe received packet.
 3. The network of claim 1, further comprising meansfor receiving a packet containing an ACK message from the destinationcomputer, for indicating receipt of the information packet by thedestination computer, and for editing the ACK message and passing thereceived packet to the source computer.
 4. The network of claim 3,wherein the editing means includes means for editing an ACK number ofthe received packet and for adjusting a checksum of the received packetin accordance with the edited value.
 5. The network of claim 1, whereinthe information packet is formatted in accordance with TCP/IP protocol.6. The network of claim 1, wherein the ACK message is formatted inaccordance with TCP/IP protocol.
 7. The network of claim 1, wherein thesatellite interface is a one-way interface.
 8. A gateway in a networksystem that forms a part of a TCP/IP network, wherein the networkincludes a source computer having a link to the TCP/IP network and alink to a high speed satellite interface, and a destination computerhaving a link to the TCP/IP network and a link to the high speedsatellite interface, the gateway comprising: means for receiving aninformation packet sent from the source computer and for sending theinformation packet to the destination computer over the satelliteinterface; and means for sending an ACK message to the source computerin response to receipt of the information packet, wherein the ACKmessage appears to the source computer to have come from the destinationcomputer.
 9. The gateway of claim 8, further comprising means forreceiving a packet containing an ACK message from the destinationcomputer, for indicating receipt of the information packet by thedestination computer, and for discarding the ACK message from thedestination computer when no other data is present in the receivedpacket.
 10. The gateway of claim 8, further comprising means forreceiving a packet containing an ACK message from the destinationcomputer, for indicating receipt of the information packet by thedestination computer, and for editing the ACK message and passing thereceived packet to the source computer.
 11. The gateway of claim 10,wherein the editing means includes means for editing an ACK number ofthe received packet and for adjusting a checksum of the received packetin accordance with the edited value.
 12. The gateway of claim 8, whereinthe information packet is formatted in accordance with TCP/IP protocol.13. The gateway of claim 8, wherein the ACK message is formatted inaccordance with TCP/IP protocol.
 14. A method for sending informationover a high speed satellite interface in a network system that forms apart of a TCP/IP network, wherein the network includes gateway and asource computer, having a link to the TCP/IP network, a destinationcomputer, having a link to the TCP/IP network, and a satellite interfacebetween the source computer, the gateway, and the destination computer,wherein information passes from the source computer to the destinationcomputer, the method comprising the steps, performed by a processor ofthe gateway, of: receiving an information packet sent from the sourcecomputer; sending the information packet to the destination computerover the satellite interface; and sending an ACK message to the sourcecomputer in response to receipt of the information packet, wherein theACK message appears to the source computer to have come from thedestination computer.
 15. The method of claim 14, further comprising thesteps of receiving a packet containing an ACK message from thedestination computer, wherein the ACK message indicates receipt of theinformation packet by the destination computer and discarding the ACKmessage from the destination computer when no other data is present inthe received packet.
 16. The method of claim 14, further comprising thesteps of receiving a packet containing an ACK message from thedestination computer, wherein the ACK message indicates receipt of theinformation packet by the destination computer, and editing the ACKmessage and passing the received packet to the source computer.
 17. Themethod of claim 16, wherein the editing step includes the substeps ofediting an ACK number of the received packet and adjusting a checksum ofthe received packet in accordance with the edited value.
 18. The methodof claim 14, wherein the information packet is formatted in accordancewith TCP/IP protocol.
 19. The method of claim 14, wherein the ACKmessage is formatted in accordance with TCP/IP protocol.